Concrete’s Compressive strength Vs time

Concrete gains strength by the hydration of cement particles. The rate of hydration is fast in the beginning, but its rate decreases as the time passes. It is observed that about 90% hydration is complete within 28 days and the rest 10% hydration takes years together.

Theoretically, concrete need water/cement ratio of theoretically 0.38 (0.23 is sufficient for actual hydration process and 0.15 is required for filling the voids in the gel pores). But practically, we need 0.4 to 0.5 as the concrete will be in closed container or heaped so that to maintain the desired RH level.

You can see the study shows the % of shrinkage of concrete Vs the time line, which gives you clear indication of chemical reaction in concrete causing the physical changes:

As per IS 456, concrete is designed / formulated to gain its maximum strength in 28 days. It may also gain further strength, but it will be very less.

70% strength is developed in about 10 days and 75% strength in 14 days. Hence the period of curing in the laboratory is recommended as 28 days and in site at least 7 days. See the % of 28 days strength gain in lab for the longtime line:

Can we find out flexural strength from the compression strength and vice-versa?

According to IS 456- 2000 Pg 16 clause 6.2.2, The flexural and splitting tensile strength shall be obtained as described in IS 516 and IS 5816 respectively. When the designer wishes to use an estimate of the tensile strength from the compressive strength the following formula may be used:

         Fcr = 0.7* √Fck

Fcr – flexural strength

Fck – characteristic strength of concrete N/mm²

With reference to : http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.680.3531&rep=rep1&type=pdf from Proc. of Int. Conf. on Advances in Design and Construction of Structures 2012, paper presents a simple mathematical model to predict the compressive strength of concrete at 28 day from early age (say 7 days) results. In this paper they concludes:

This paper simplifies the mathematical model to estimate the 28 days compressive strength of concrete from just only one parameter, 7 days test results without considering the other index properties of concrete [e.g. water, cement, w/c ratio, FA, CA, density (ρ)] and gives an initial idea of the 28 days strength with reasonable accuracy. In this study, modeled concrete strength gain characteristic with age is simplified and a simple mathematical (power) equation is introduced which replaces the polynomial surface equation. The proposed equations predicts well the 28 days strength for concrete made with stone aggregate and has potential to predict the strength of concrete made with brick aggregates. The proposed technique can be used as a reliable tool for assessing the design strength of concrete from quite early age test results. Besides predicting 28 days strength for ordinary Portland cement concrete, expected concrete strength at any age (say 21, 60 or 360 days) can be determined from the proposed model.

Let see the animated LAB testing video for better understanding:

Many thanks to google.com / engineeringenotes.com, and 

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